perf: single-pass contiguous UPDATE for plaintext fixed-width tables

[MPCoreDeveloper]

Summary

B8: single-pass contiguous UPDATE fast path. When a batch UPDATE (via ExecuteBatchSQL → Table.UpdateMultiple) hits a plaintext fixed-width columnar table with strictly ascending pk = <literal> matches whose records are physically adjacent in the data file (fixed-width ⇒ constant stride [4-byte length][FixedSize payload]), the target records are read as one contiguous byte range through the storage layer's cached file handle and patched in memory — no per-row pread.

Measured (AppendOnly --pk, default = fixed-width)

	before	after
UPDATE	~45K ops/s	~84K ops/s (+86%)
UPDATE gap vs SQLite	5.1×	3.2×

Correctness & scope

• Strictly gated; any deviation falls back to the generic per-row loop before any record is touched:
  • fixed-width + Columnar + explicit PK; plaintext only (NoEncryptMode) so a raw range equals the logical records;
  • no buffered overwrites for the file (new IStorage.HasBufferedOverwrite) so the range read can never see stale disk bytes;
  • keys strictly increasing, every position verified adjacent (B-tree), every 4-byte length prefix verified;
  • no PK writes, no CHECK constraints; fixed-size hash-indexed SET columns are re-pointed in-batch (old value decoded from the raw slot); variable-length (TEXT/BLOB) indexed columns and encrypted DBs fall back.
• New IStorage.ReadBytesRange (default null; Storage implements it over the cached read handle — ReadBytesAt's fresh per-call handle conflicted with the transaction's open handles).
• New Table.BulkContiguousUpdateBatches diagnostics counter; the tests prove the path engages.

Tests

Full suite 1650/1650 (+5 new FixedWidthBulkUpdateTests): engage + durability across reopen, re-engage on a second committed batch (fresh bytes), fallback for gapped keys, fallback for hash-indexed/variable columns with index re-point correctness, fallback for non-plaintext config.

Note: a pre-existing quirk surfaced while writing these tests — a large descending-order ExecuteBatchSQL UPDATE batch silently applies nothing in the generic loop (legacy and fixed-width). Out of scope here; the fallback test uses a gapped-ascending shape instead.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud